Fuel cells have been used as a power source in many applications. For example, fuel cells have been proposed for use in electrical vehicular power plants to replace internal combustion engines. In proton exchange membrane (PEM) type fuel cells, hydrogen is supplied to the anode of the fuel cell and oxygen is supplied as the oxidant to the cathode. PEM fuel cells include a membrane electrode assembly (MEA) comprising a thin, proton transmissive, non-electrically conductive, solid polymer electrolyte membrane having the anode catalyst on one face and the cathode catalyst on the opposite face. The MEA is sandwiched between a pair of non-porous, electrically conductive elements or plates which serve as current collectors for the anode and cathode, and contain appropriate channels and/or openings formed therein for distributing the fuel cell's gaseous reactants over the surfaces of the respective anode and cathode catalysts.
The electrically conductive plates sandwiching the MEAs may contain an array of grooves in the faces thereof that define a reactant flow field for distributing the fuel cell's gaseous reactant's (i.e., hydrogen and oxygen in the form of air) over the surfaces of the respective cathode and anode. These reactant flow fields generally include a plurality of lands that define a plurality of flow channels or passages therebetween through which the gaseous reactants flow from a supply header at one end of the flow channels to an exhaust header at the opposite end of the flow channels.
Typical start-up and shutdown procedures for a fuel cell stack have included at start-up, introducing hydrogen gas to the previously purged anode passages and providing an oxidant, such as air, to the cathode passages. At shutdown, the supply of hydrogen to the anode passages is ceased and an oxidant, such as air, is used to purge the remaining hydrogen from the anode flow passages so that the fuel cell is unable to generate electricity. The oxidant introduced to the anode passages also removes any remaining build-up of water that is in the flow passages in order to prevent freezing of the water during shutdown. In addition, an oxidant is also used to purge the cathode passages, similarly to remove any additional moisture from the passages. Recently, it has been determined that MEA degradation can occur due to this start-up and shutdown procedure.
Without intending to be limited by theory, it is believed that the major reason for degradation of the MEA is due to the diffusion of air into the anode side forming an H2/air front while the cathode passages are still filled with air. With the presence of the H2/air front at the anode, it results in a short circuited fuel-cell between the H2/air front that generates ion current through the high lateral ionic resistance of membrane producing a significant lateral potential drop in the membrane. This lateral potential drop causes a cathode potential of 1.5 volts versus the local electrolyte. This elevated cathode potential results in corrosion of the carbon support material in a cathode catalyst causing significantly irreversible cell performance degradation. The cell performance degradation is illustrated in FIG. 3 which shows a graph of these cells voltage decrease over 200 cycles of the fuel cell stack, each cycle including a start-up and shutdown of the fuel cell stack. A cell voltage represented by line 100 is shown to decrease from 0.8 volts at the start to approximately 0.7 volts after 200 cycles. The reduction in the cell voltage over the 200 cycle period clearly indicates that the cell degraded over the 200 cycle test time.